In known mixers of this type, particularly in so-called ploughshare mixers, that is to say in mixers whose mixing mechanism has plough-share-like mixing tools, there is a risk of a deposit of the material being mixed building up on the internal wall of the mixing container. If this material contains interreacting components, the formation of such a deposit is particularly undesirable since the deposit can harden to such an extent that it can only be removed with a considerable amount of time-consuming effort. The hardened deposit causes abnormally high wear of the tools and, in extreme cases, can even cause the mixing mechanism to jam or prevent the mixing mechanism from starting again after a stoppage if the friction between the outer edge of the mixing tools and the deposit on the wall exceeds the start-up power of the drive mechanism which operates the mixing mechanism.
The formation of a deposit on the internal wall of the mixing container is also undesirable because portions of such deposit break off during the mixing process and fall into the material being mixed and this leads to inhomogeneities in the material. If the mixing container is provided with wall heating means, deposits of the material being mixed on the wall prevent the transfer of heat from the wall heating means into the interior of the mixing container, and the thicker the layer deposited the greater the extent to which it occurs.
Deposits of the material being mixed are able to form on the internal wall of the mixing container because the mixing tools have to revolve at a certain distance from the internal wall of the mixing container in order to ensure that there is no contact between the mixing tools and the container wall during operation.
Although the formation of a deposit can be substantially suppressed by special shaping of the mixing tools and in particular of their edges revolving in the vicinity of the container walls, the formation of a deposit cannot be avoided completely when some materials are mixed. In the past, the formation of such deposits could only be dealt with effectively by cleaning the mixing container frequently by a process involving manually detaching any deposits adhering on its internal wall and removing them from the mixing container. However, this necessitates long stoppages and high labour costs which substantially increased the operating costs of the mixer, and was therefore undesirable. In particular, a cleaning process of this type often does not meet current regulations governing operational safety and the measures protecting personnel.